Direct Image by Geoprobe

HPT Equipment. Flow to the HPT probe is controlled by the HPT controller,
which also sends data to the Field Instrument for real-time display and storage.

HPT System Schematic. Water is pumped through
the trunkline and out the screen; the resulting
soil pressure response is measured by the downhole
transducer.

HPT is designed to evaluate the hydraulic behavior of unconsolidated materials. As the probe is pushed or hammered at 2 cm/s, clean water is pumped through a screen on the side of the HPT probe at a low flow rate, usually less than 300 mL/min. Injection pressure, which is monitored and plotted with depth in real time, is an indication of the hydraulic properties of the soil. That is, a relatively low pressure response would indicate a relatively large grain size and the ability to easily transmit water. A relatively high pressure response, however, would indicate a relatively small grain size and the lack of ability to transmit water. The HPT system operates using principles similar to those of Dietrich (2003). Since the pressure transducer is located downhole, the HPT system also can be used to collect profiles of static water pressure data, which can be used to calculate static water levels.

A Wenner array is integrated into the HPT probe. This allows a user to collect soil electrical conductivity (EC) data for lithologic interpretation (Christy, 1994). In general, the higher the electrical conductivity value, the smaller the grain size, and vice versa. However, other factors can affect EC, such as mineralogy and pore water chemistry (brines, extreme pH). Conversely, the HPT pressure response is independent of the chemical and mineralogical factors.

Since the HPT pressure response is analogous to relative changes in the ability to transmit water (and therefore the relative change in dominant grain size), the HPT system can be used to identify potential contaminant migration pathways. Similarly, it can help identify zones for remedial material injection or provide qualitative guidance on how difficult injection may be in different zones of the formation.

Since the HPT pressure response is analogous to relative changes in the ability to transmit water (and therefore the relative change in dominant grain size), the HPT system can be used to identify potential contaminant migration pathways. Similarly, it can help identify zones for remedial material injection or provide qualitative guidance on how difficult injection may be in different zones of the formation.